Method and apparatus for balancing power plane pin currents in a printed wiring board

ABSTRACT

A method and apparatus for balancing power plane pin currents in a printed wiring board (PWB) provides for reduction in pin counts required for power plane (including ground plane) connections and/or reduction in requirements for connector current handling per pin. One or more slots is introduced in the metal layer implementing the power plane that alter the current distribution in the power plane. The per-pin current profile for connector pins connected to the power plane is equalized by tuning the length of the slot(s). The slots may be dashed or made internal to the power plane metal layer to avoid weakening the metal layer for laminated multi-layer PWBs and may be shaped around a connector end when the power plane pin allocation is not uniform at the connector ends. The resulting equalization reduces either pin count required for carrying the power plane current or reduces connector pin current requirements.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to electrical and electroniccircuits and systems fabricated on printed wiring boards and morespecifically to a method for balancing power plane pin currents andprinted wiring board having balanced pin currents.

2. Description of the Related Art

Printed wiring boards (PWBs), also referred to as printed circuit boards(PCBS) have been in use for decades for fabricating circuits and entiresystems. The PWBs provide the interconnects for discrete and integratedcomponents and subsystems and provide power paths or power planes forinterconnecting the components to power supplies.

Power distribution in PWBs has always been a concern and in particular,high current systems such as today's processing systems andinterchangeable processing sub-units (“blades”) require the handling ofvery high currents per PWB on some power supply connections, which cangenerate substantial voltage drops within the PWB conductor(s) andrequire multiple connector pins or other connector contacts connected inparallel to carry the amount of current supplied to a particular powersupply distribution net.

To alleviate the voltage drop problem (and also provide electromagneticshielding), processing systems and subsystems integrated on a PWBtypically use specific layers of a multilayer PWB to carry the powersupply voltages and returns or may include a few other connections, butwill primarily be power supply layers. A layer dedicated to power supplydistribution may actually include multiple power planes distributing twoor more separate power supply outputs or may be dedicated todistributing a single power supply output.

The large metal areas typically used for power planes reduces thevoltage drop to the connector pins or other terminals used to connectthe PWB to a power supply. However, differential voltages exist betweenthe power supply terminal connections, even with a continuous metalplane, because of differing resistive path lengths between the terminalsand the current sinks or sources (e.g., a large current consumer such asa processor) and the individual terminals. Additionally, the currentdistribution in the power plane metal, which is not uniform, contributesto the differential voltages between the terminals, and the differentialvoltages cause non-uniform distribution of terminal currents. Ingeneral: 1) terminals that are closer to the current sources and sinks(i.e., the device power terminals) on the PWB carry more current due tothe shorter paths; and 2) terminals that are toward the outside of theterminal array carry higher currents due to the decreased currentdensity away from the center of the connector length (because of loweredvoltage drop per unit length along the paths passing through lowercurrent density regions). Both of the above-recited factors superimposeto yield a particular terminal current distribution for each power planeand for each PWB/terminal configuration.

In present-day systems, such as large scalable server systems operatingat relatively low voltages, the current levels per PWB and per-terminalare very large. As such, a significant amount of power is dissipated inthe connectors due to pin resistance and in the PWBs themselves due tothe finite conductivity of the metal layers used to implement the powerplanes. The use of thin laminated PWBs having many layers also increasesthe effective resistance of the paths between devices on the PWB and theconnector terminals, leading to an increase in the terminal currentdistribution described above.

The disparity in terminal currents leads to a need to over-specify aconnector for pin current handling, which is typically set by themaximum power dissipation through the pin and the overall tolerable pinresistance (dictated by the maximum voltage drop(s) to the components onthe PWB). Alternatively, an increase in the total number of terminalsrequired to couple the power supply to the PWB power plane(s) isrequired.

Also, overall power dissipation is increased by a disparate terminalcurrent distribution. Because the power dissipation per terminal (bothin the power plane and the connector pin) is a function of the square ofthe current through the terminal, the average power dissipation in aconnector is not constant over all the possible terminal currentdistributions, but is at a minimum when the terminal currentdistribution is equal. For example, for two terminals carrying a totalof 4A, if the pin currents are equal, the power dissipation in watts is8R where R is the resistance of the pins. If the pin currents are 1A and3A respectively, the power dissipation in watts is 10R. Equalizing theterminal current distribution minimizes the power dissipation in theconnector, as well as generally minimizing average power dissipation inthe power plane metal area.

It is therefore desirable to provide a method for PWB power plane designand a PWB power plane implementation that equalizes the connectorterminal currents for power supply connections.

SUMMARY OF THE INVENTION

The objective of equalizing terminals currents at a PWB connection isprovided in a method for PWB design and a PWB design that alters powerplane current distribution to equalize the current distribution amongmultiple terminals commonly connected to the power plane. The method maybe embodied in a computer program product for PWB design.

A slot is introduced in the power plane or between two power planes toalter the current distribution in the power plane(s) so that currentthrough power supply connection terminals commonly connected to thepower plane(s) is equalized.

A slot may be introduced beyond each physical end of a connector and theslot lengths tuned to equalize the currents between the power plane andterminals of the connector connected to the power plane. The slot may bedashed to reduce structural weakening at the edge of the PWN for thinlaminated PWBs, in particular to reduce bending or tearing duringlamination of metal layers with dielectric layers. The slot may also beshaped around a connector end when connector column lengths are notequal or when the power plane terminals are not evenly distributedacross rows of the connectors at an end of a connector.

Multiple power planes within a single layer of a PWB may have connectioncurrents tuned in the above manner and/or multiple layers having powerplanes within a single PWB may be likewise tuned.

The foregoing and other objectives, features, and advantages of theinvention will be apparent from the following, more particular,description of the preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives, and advantages thereof,will best be understood by reference to the following detaileddescription of an illustrative embodiment when read in conjunction withthe accompanying drawings, wherein like reference numerals indicate likecomponents, and:

FIGS. 1A and 1B are current distribution diagrams comparing a currentdistribution of a prior art PWB with a PWB designed in accordance withan embodiment of the invention.

FIG. 2 is a pictorial diagram depicting a PWB power plane layer inaccordance with an embodiment of the invention.

FIGS. 3A, 3B and 3C are pictorial diagram showing details of power planelayer designed in accordance with other embodiments of the invention.

FIG. 4 is a pictorial diagram depicting a multi-power plane PWB layerdesigned in accordance with another embodiment of the invention.

FIG. 5 is a flowchart depicting a method in accordance with anembodiment of the present invention.

FIG. 6 is a pictorial diagram depicting a workstation computer system inwhich the method of FIG. 5 can be practiced by executing programinstructions of a computer program product in accordance with anembodiment of the present invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

With reference now to the figures, and in particular with reference toFIGS. 1A and 1B, advantages of the techniques of the present inventionare illustrated by pictorial diagrams that show the current distributionbetween terminal lands 12 of a connector mounting area 10A, 10B. In FIG.1A, a standard PWB current distribution is shown, with the brightness ofterminal lands 12 indicating the current level at that terminal when theconnector terminals are commonly connected to a power supply output. Asolid copper layer is generally used to implement a power plane(including ground planes) and the voltage rise at terminal lands 12 istypically higher toward the center of the connector or in locationswhere the highest current consumers (or providers for return planes) arelocated. For this example, a high current consumption is assumedparallel to the center of the connector, but the techniques of thepresent invention may be applied to any terminal current distribution.The increased voltage rise leads to less actual current being providedor accepted by the power supply output as shown by the darker (lowercurrent) terminals, resulting in an unequal distribution of currentamong the power supply terminals at a connector. The amount of currentdrawn by a terminal is a function of both the path length and thecurrent density along the path from the power supply connection of thedevices mounted on the PWB to the terminal. Since the current density ishigher toward the center of the connector, but the path length is lower(assuming the device are centered on the PWB), the lowest currentterminals are disposed toward the center of the connector and toward theedge of the connector mounting area farthest from the devices, as shownin FIG. 1A.

It is highly desirable to equalize the current distribution among theterminals, as the maximum current per terminal generally determines theminimum number of connector terminals for a given power plane.Alternatively, or partially in conjunction, the minimum current handlingfor the terminal pins, maximum voltage rise for a terminal pin orwiring/backplane from the power supply if individual runs are employed,and the individual current handling of individual runs can be relaxed ifthe terminal current distribution is equalized. Further, due to thesquare-law relationship of power vs. current, overall power dissipationin the connector and the PWB is reduced when terminal currents areequalized.

FIG. 1B shows an equalized current distribution across terminals 12 inconnector mounting area 10B of a PWB in accordance with an embodiment ofthe invention. The overall shading of terminals 12 compared to terminals12 in FIG. 1B shows that the current per pin has been equalized overthat of FIG. 1A. The connector terminal lands in the central rows ofconnector mounting area 10B, and in particular those farthest away fromthe PWB devices (toward the left of the figure) are carrying morecurrent than the same terminals in the PWB illustrated by FIG. 1A. Theterminal lands toward the ends of connector mounting area 10B arecarrying less current in the PWB illustrated by FIG. 1B than in the PWBillustrated in FIG. 1A. In general, the minimum and maximum terminalcurrents are much closer in FIG. 1B than for the terminal currentsdepicted in FIG. 1B and therefore, the power dissipation in theconnector and PWB is significantly lower. The reduction of the maximumterminal current permits increased terminal current handling margins,reduced terminal specification requirements and/or a reduced number ofterminals required for the power plane connections.

Referring now to FIG. 2, a power plane 20 (single metal layer) of PWB inaccordance with an embodiment of the invention is shown. Two slots 22Aand 22B are introduced in power plane 20 that alter the current densitynear the ends of connector mounting area 24 and having tuned lengths L1and L2, respectively that are adjusted to substantially equalize theconnector terminal currents. Other features of power plane 20 includethrough holes 26B for passage of signal and other connections notconnected to power plane 20, and power plane terminals 26A for receivingblind vias or leads that connect to power plane 20. Also illustrated isan integrated circuit package mounting area 28 such as a processor thatis a high current consumer that generates the “current crowding” problemsolved by the present invention.

While FIG. 2 illustrates only one power plane 20, it should beunderstood that the technique applied to power plane 20 can be appliedto every power plane in a PWB, permitting equalization of terminalcurrents in groups corresponding to each power supply output connectedto an associated PWB power plane. The slot lengths can be determinedindividually for each power plane based on the amount of correctionneeded for the particular plane in order to equalize the terminalcurrents for each and all planes.

Referring now to FIG. 3A, another type of slot 22C that can be used toalter current density and equalize terminal current distribution isshown. The dashed line form of slot 22C is used to avoid reducing thestrength of a metal layer, especially toward the edges, where tearingmay occur during handling or lamination of built-up PWBs.

Referring now to FIG. 3B, yet another type of slot 22D that can be usedto alter current density and equalize terminal current distribution isshown. The bent line form of slot 22D is used to permit critical signaltraces 30 to be routed around the ends of connector mounting area 24, asthe impedance and electromagnetic coupling of signal trace 30 would beaffected by a slot such as slot 22A of FIG. 2, which would cross signaltrace 30.

Referring now to FIG. 3C, still another type of slot 22E that can beused to alter current density and equalize terminal current distributionis shown. The jogged line form of slot 22E is used when the terminals ofa connector are unused (as depicted, there are no connections to twolocations), or when the power plane terminals in the row(s) at the endof the connector are not evenly distributed. The jog in the line altersthe current distribution in both directions of the connector, permittingequalization of the terminal current distribution even when theterminals are not evenly distributed to the power plane.

Referring now to FIG. 4, a metal layer 40 of a PWB having multiple powerplanes on a single layer is shown. Such a PWB is illustrative of anapplication where there are multiple processors on a PWB, each having anindividual power supply output providing their power. Rather than slotsplaced at ends of the connector, the multi-power plane solution of thepresent invention provides for use of a gap 42 between the power planesas a tuning mechanism for altering current distribution among theterminals connecting each power plane to its corresponding power supplyoutput. The position and/or width of gap 42 can be altered, as shown, tomake changes in the current distribution of each power plane whererequired, yielding an equalized terminal current distribution for eachof the power planes. Additionally, slots such as slot 22F having a tunedlength as described above with respect to FIG. 2, may be used toequalize the terminal current distribution(s) on one or more of thepower planes with respect to the connected terminals.

Referring now to FIG. 5, a method in accordance with an embodiment ofthe present invention is depicted. First, a PWB metal layer is set upfor simulation by introducing one or more slots (step 50). Next, thecurrent and potential across the metal are simulated for known circuitconditions (step 52). Then, from the distributions computed in step 52,the terminal current distribution is determined (step 54) and thedistribution is tested for equality and/or convergence criteria(decision 56). If the criteria are not met, the slot lengths areadjusted (step 58), a slot may also be removed, added or relocated andthen steps 50-56 are repeated. Other techniques may be used inaccordance with other embodiments of the invention, includingdetermination of the current/potential distribution by actualmeasurement and/or calculation of initial slot dimensions from empiricalmodels applied to actual or simulated terminal current distributions orcondensed statistics such as the current deviation. In general, thetechniques of the present invention provide a mechanism via the slots,by which the current distribution is equalized as best as possible bytuning the slot lengths.

Referring now to FIG. 6, a workstation computer system, in which methodsaccording to an embodiment of the present invention are performed, isdepicted. A workstation computer 112, having a processor 116 coupled toa memory 117, for executing program instructions from memory 117,wherein the program instructions include program instructions forexecuting one or more methods in accordance with an embodiment of thepresent invention.

Workstation computer 112 is coupled to a graphical display 113 fordisplaying program output such as simulation results and power plane andcircuit layout structure input, design and verification programsimplementing embodiments of the present invention. Workstation computer112 is further coupled to input devices such as a mouse 115 and akeyboard 114 for receiving user input. Workstation computer may becoupled to a public network such as the Internet, or may be a privatenetwork such as the various “intra-nets” and software containing programinstructions embodying methods in accordance with embodiments of thepresent invention may be located on remote computers or locally withinworkstation computer 112.

While the invention has been particularly shown and described withreference to the preferred embodiment thereof, it will be understood bythose skilled in the art that the foregoing and other changes in form,and details may be made therein without departing from the spirit andscope of the invention.

1. A method for controlling a current distribution among a plurality ofterminals commonly connecting a power plane of a printed wiring board toa power supply, said method comprising: providing a metal layercomprising said power plane, said layer comprising at least one slotproximate the terminals for altering a current flow in the vicinity ofsaid at least one slot; determining a profile of said currentdistribution among said terminals; and adjusting a length of said atleast one slot to equalize said current distribution among saidterminals.
 2. The method of claim 1, wherein said determining and saidadjusting are iterated until said current distribution is substantiallyequal.
 3. The method of claim 1, wherein said at least one slot consistsof two slots, one disposed at a first end of a connector bearing saidplurality of terminals and one disposed at a second end of saidconnector and wherein said adjusting adjusts a length of each of saidtwo slots to attain a substantially equal current distribution amongsaid plurality of terminals.
 4. The method of claim 1, wherein saidplurality of terminals are terminals of a multi-row connector, whereinsaid plurality of terminals are not evenly distributed across rows ofsaid connector at an end of said connector, wherein said at least oneslot is disposed at an end of said connector and further comprisingshaping said slot to improve equality of current distribution acrosssaid rows at said end of said connector.
 5. The method of claim 1,wherein said at least one slot is a plurality of colinear slots in theform of a dashed line, and further comprising laminating said metallayer with other layers of said PWB, whereby said dashed line improves astrength of said metal layer to avoid damaging said metal layer duringsaid laminating.
 6. The method of claim 1, further comprising: providingat least one other metal layer comprising one or more other powerplanes, said at least one other layer each comprising at least one otherslot for altering a current flow in the vicinity of said at least oneslot; determining a profile of said current distribution for each ofsaid at least one other metal layers; and adjusting a length of said atleast one other slot to equalize a current distribution in terminalsconnecting each of said other metal layers to a respective power supplyoutput.
 7. The method of claim 1, wherein said metal layer comprises atleast one other power plane, and wherein said at least one slot definesa boundary between said power plane and said at least one other powerplane, and wherein said adjusting adjusts a shape of said at least oneslot to equalize said current distribution and to further equalize acurrent distribution in terminals connecting said at least one otherpower plane layers to a power supply output.
 8. A printed wiring board(PWB), comprising: at least one power plane comprising a large metalarea for connecting power connections of circuits mounted on said PWB toa power supply; and a plurality of terminals for providing connection ofsaid power plane to said power supply, and wherein said power planedefines at least one slot proximate the terminals and having a tunedlength for altering a current density of the power plane in the vicinityof said at least one slot in order to equalize currents flowing throughsaid terminals.
 9. The printed wiring board of claim 8, wherein said atleast one slot consists of two slots, one disposed at a first end of aconnector mounting area containing said plurality of terminals and onedisposed at a second end of said connector mounting area, and whereineach of said slots has a unique tuned length to attain a substantiallyequal current distribution among said plurality of terminals.
 10. Theprinted wiring board of claim 8, wherein said plurality of terminals areterminals for mounting a multi-row connector and wherein said pluralityof terminals are not evenly distributed across rows of said connector atan end of said multi-row connector, wherein said slot is disposed atsaid end of said connector, and wherein said slot is shaped to improveequality of current distribution across said rows at said end of saidconnector.
 11. The printed wiring board of claim 8, wherein said atleast one slot is a plurality of colinear slots in the form of a dashedline, and wherein said power plane is laminated with other layers ofsaid PWB with dielectric layers between, whereby said dashed lineimproves a strength of said power plane to avoid damaging said powerplane during handling and lamination.
 12. The printed wiring board ofclaim 8, further comprising at least one other power plane, each of saidpower planes comprising a large metal area defining at least one otherslot for altering a current flow in the vicinity of said at least oneslot, and wherein each of said at least one other slot has a uniquetuned length adjusted to equalize a current distribution in terminalsconnecting each of said other metal layers to a respective power supplyoutput.
 13. The printed wiring board of claim 8, wherein said PWBcomprises at least one other power plane comprising a second large metalarea located on the same layer as said at least one power plane, andwherein said at least one power plane and said at least one other powerplane define said at least one slot by a boundary gap between said atleast one power plane and said at least one other power plane, andwherein a shape of said at least one slot is tuned to equalize saidcurrent distribution and to further equalize a current distribution interminals connecting said at least one other power plane to a powersupply output.
 14. A computer program product comprising media encodingprogram instructions for execution on a workstation computer, saidprogram instructions for determining a shape of one or more metal layersof a printed wiring board (PWB), wherein said program instructions areadapted for controlling a current distribution among a plurality ofterminals commonly connecting a power plane of at least one of the metallayers to a power supply, said method comprising: introducing a slot insaid at least one metal layer and proximate said terminals for alteringa current flow in the vicinity of said at least one slot; determining aprofile of said current distribution among said terminals; and adjustinga length of said at least one slot to equalize said current distributionamong said terminals.
 15. The computer program product of claim 14,further comprising program instructions for repeating said determiningand adjusting until said current distribution is substantially equal.15. The computer program product of claim 14, wherein said programinstructions for introducing introduce two slots, one disposed at afirst end of a connector mounting area bearing said plurality ofterminals and one disposed at a second end of said connector mountingarea and wherein said program instructions for adjusting adjust a lengthof each of said two slots to attain a substantially equal currentdistribution among said plurality of terminals.
 16. The computer programproduct of claim 14, wherein said plurality of terminals are terminalsof a multi-row connector, wherein said plurality of terminals are notevenly distributed across rows of said connector at an end of saidconnector, wherein said at least one slot is disposed at an end of saidconnector and further comprising program instructions for shaping saidslot to improve equality of current distribution across said rows atsaid end of said connector.
 17. The computer program product of claim14, wherein said program instruction for introducing introduce aplurality of colinear slots in the form of a dashed line, whereby saiddashed line improves a strength of said at least one metal layer toavoid damaging said metal layer during said laminating.
 18. The computerprogram product of claim 14, wherein said at least one metal layer is aplurality of metal layers and wherein said program instructions forintroducing introduce at least one slot in each of said plurality ofmetal layers and wherein said program instructions for determining andadjusting operate on each of said plurality of metal layers to equalizeconnector terminal distributions for each of several power supplyoutputs connected to corresponding metal layers.
 19. The computerprogram product of claim 14, wherein said at least one metal layerincludes a single metal layer with multiple power planes, and whereinsaid at least one slot defines a boundary between said multiple powerplanes, and wherein said program instructions for adjusting adjust ashape of said at least one slot to equalize current distribution amongterminals in each group of terminals connecting said multiple powerplanes to corresponding power supply outputs.
 20. A printed wiring board(PWB), comprising: at least one power plane comprising a large metalarea for connecting power connections of circuits mounted on said PWB toa power supply; a plurality of terminals for providing connection ofsaid power plane to said power supply; and means incorporated withinsaid at least one power plane for equalizing a current distributionamong said terminals.